This invention relates to extruders, and in particular, to a method and apparatus for the continuous extrusion of porous activated carbon block filter elements.
Screw extruders and injection molding machines are well known in the art. A significant disadvantage of the current state of the art methods and devices for the extrusion of composite articles including a resin binder and powdered or granular material is that, as the material flows off the screw and into the die, and begins to cool and harden, the flights extending from the screw create a spiraling "knit mark" along the outer surface of the material. This spiraling "knit mark" along the surface of the material results in a spiraling weak area all along the axial length of the material which extends through the body of the material and which has a lower density and a greater porosity. Product strength and filtering capability are both adversely affected.
In order to compensate for the lower density and the greater porosity along the spiraling weak area of the material, current state of the art methods and devices form the product with a greater overall density. This, in turn, causes higher production costs due to increased material usage, lower production rates, increased wear and tear on the extruder apparatus, and higher power consumption.
Attempts to overcome the disadvantages of current state of the art extrusion methods and devices are disclosed in the Koslow U.S. Pat. No. 5,189,092 and the Koslow U.S. Pat. No. 5,249,948. The Koslow '092 patent and the Koslow '948 patent disclose an apparatus and a method to provide a more uniform extruded product. The Koslow patents disclose passing the pondered material through the extruder barrel by a screw feeder. The material in the extruder is preheated in the barrel by a preheater, and thereafter passed into a die where it is further heated by a heater element to the required processing temperature.
The heating of the material in the die is critical. If the heating of the material flowing through the die is too short, and the flow of the material too fast, it is not possible to transmit the heat required to form a solid object and the material may not solidify properly. On the other hand, if the heating section is too long, the back pressure increases uncontrollably so that the material hardens and locks up the extruder. Furthermore, the cooling of the material must be long enough such that the material retains its structural integrity when it emerges from the die without causing an uncontrolled amount of back pressure that can lead to material lockup.
However, as with other prior art extrusion devices, the flights of the screw create a spiraling "knit-mark" along the outer surface of the extruded block. Hence, the extruded block has a weakened area along its entire axial length. This weakened area, as previously described, has a lower density and a greater porosity.
Therefore, it is a primary object and feature of the present invention to provide a method and apparatus for the continuous extrusion of a block element which eliminates the spiraling "knit mark" along the outer surfaces of the block element.
It is a further object and feature of the present invention to provide a method and apparatus for the continuous extrusion of a block element which reduces the amount of material required, increases production rates, and reduces power consumption.
It is still a further object and feature of the present invention to provide a method and apparatus for the continuous extrusion of a block element which produces a product of uniform density and porosity.
In accordance with the present invention, a method and apparatus are provided for the continuous extrusion of a block element. The apparatus includes a hollow, generally cylindrical barrel extending along a longitudinal axis. The barrel has an input port, a discharge port, and screw receipt cavity therebetween.
A screw is positioned within the screw cavity receipt coaxial with the barrel, and is rotatable about the longitudinal axis. A heater is positioned between the input port and the discharge port for heating the barrel to a predetermined temperature. A die assembly extends the longitudinal axis adjacent to the barrel. The die assembly has an input port in communication with the discharge port of the barrel, and a discharge port. A water cooling jacket is positioned about the die assembly for cooling the die assembly to a predetermined temperature.
The method of the present invention includes providing a premixed material and feeding the material into the barrel. While in the barrel, the material is heated such that the material forms an agglomerated mass. The agglomerated mass is agitated and compacted into the die by a generally flat surface where the helical flight about the screw terminates. The agglomerated mass is cooled so as to form an extruded block.
In order to control the pressure within the die, the die pressure is monitored and the flow rate of the material exiting the die is controlled. This is accomplished passing the extruded block which exits the die between first and second wheels. Thereafter, a braking force is applied to the wheels to limit the flow of material therebetween which, in turn, varies the pressure.